Reducing the spread of HIV among intravenous drug users, and from intravenous drug users through heterosexual and perinatal transmission, is one of the most urgent public health problems in the United States and many other countries. Unfortunately, immediate success, in terms of a vaccine, a cure for HIV infection, or an infallible behavior change program is extremely unlikely. AIDS prevention programming must be done on a long term basis, and will require understanding the long term dynamics of HIV transmission among and from IV drugs users. The proposed research will provide data on long term dynamics by extending our studies of AIDS among IV drug users in Manhattan, New York City, for an additional five years. This research will also be part of a world Health Organization collaborative study that will use standardized methods to compare seroprevalence and risk behavior trends among 11 different cities. Specific aims are to determine levels of and trends in AIDS risk behavior and HIV seroprevalence among IV drug users in southern Manhattan, to study levels of and trends in immunologic status that may influence infectivity and thus the extent of viral transmission; to understand differences in HIV seroprevalence and AIDS risk behavior to ongoing AIDS prevention activities; and to compare trends in seroprevalence and risk behavior among IV drug users in southern Manhattan to those in the other cities participating in the World Health Organization study. Southern Manhattan will be the location for the study because, for this location, seroprevalence data exist since the beginning of the epidemic in the 1970s; data on relevant behavioral and immunologic parameters have been collected since 1984; the epidemic is highly advanced among IV drug users so it provides a model for possible future developments in other cities; and the ethnic mix of IV drug users in the area allows interracial comparisons. 1000 IV drug users entering treatment or recruited in street settings will be interviewed each year using a standardized sampling design and standardized questionnaire developed for the WHO study (and supplemented to capture important aspects of New York City drug culture). HIV antibody status and immunologic markers will be measured for subjects who consent to have phlebotomy performed. Analyses will examine cross-sectional differences in demographic groups; change over time in risk behavior, relationships between specific behaviors and the probability of being infected, seroprevalence, and immunologic status; and compare risk behavior and seroprevalence in New York City to those in other cities in the WHO study (in the context of differences in AIDS. and drug-related policies and prevention programs in these cities).